Alastair Campbell is busy promoting his latest political diaries, but if you email him a question about depression, he gets back to you in half an hour. "I ended up in hospital when I had my breakdown," he says. "Up until then I had resisted all attempts to persuade me that I needed help. When people started to write about this side of my life I made a conscious decision always to be open and to hope that people who have a bad breakdown can see it does not have to mean your life is over."

Mental health needs this kind of public discussion. A report published last week by the London School of Economics said that a third of families have a member with mental health problems but only one in four people who need treatment are getting it. This is partly due to people not realising that they have got a mental illness but also because of a lack of resources to treat them.

The most common mental health problems are depression and anxiety, which often co–exist. The report points out that other mental health problems such as schizophrenia are also under treated but are rarer and a higher proportion of people get help. Depression is a relapsing condition for half of people who have it, but the other half can be cured

How can you tell if you have depression?

It seems that few of us diagnose ourselves as depressed. "People often come to see me not with a mental health problem, but with the impact it has on their lives," says Dr Andrew Jones, a sessional GP in Surrey and head of clinical engagement at the BMJ Group. "They'll be struggling at work, not sleeping, drinking too much alcohol or getting headaches. But they know somewhere that it's not really the problem."

This may be because it is hard to recognise your own depression. "We all know what it is like to feel sad when something bad happens but with depression the feelings go on for much longer and you don't bounce back," says Dr Jim Bolton, a psychiatrist in south-west London. "If you think you are depressed or anxious you should see your GP. It won't be an uncomfortable conversation."

In fact GPs can detect depression by running through a short questionnaire, the PHQ-9, in which you are asked if you are feeling tired, having difficulty concentrating or if you have thought about ending your life.

What causes depression?

There is no neat theory to explain depression. "A lot of sadness is entirely appropriate," says Dr Bolton, "but depression seems to carry on when the events have resolved. We all have a certain resilience and vulnerability. One person might see something as a challenge while another will feel overwhelmed by it and that will depend on their life experiences, upbringing and genetic factors."

Helen Lester, GP and professor of primary care at the University of Birmingham says: "We don't know enough about depression to explain it. The evidence is unclear about the interaction between genes and the environment."

How do you know if you need treatment?

"Depression is divided into mild or moderate and severe," says Lester. "If someone has mild depression, I talk about changes they can make to their lifestyle such as reducing how much they drink, advising them to sleep properly and to exercise. (The National Institute for Health and Clinical Excellence recommends three 45- to 60-minute sessions a week although recent research suggests this won't necessarily help.) They can also be prescribed online cognitive behavioural therapy to do once a week for two to three months. It is quite hard work: people have to note their negative thoughts and learn how to think about positive things."

If the depression is more severe, antidepressants are suggested, such as selective serotonin re-uptake inhibitors (SSRIs), eg Prozac or Citalopram, which increase the amount of serotonin in the brain and seem to help lift your mood. "I rarely prescribe on the day," Lester says. "I would ask you to look up antidepressants and the side-effects, for example 5% will get diarrhoea, and to chat to friends and family about whether you should take them. Then a week later I would ask you to come in for a review. Often women in particular will say they hadn't realised some of their friends had been on them. This normalises it – mental illness is not this terrible scary thing that means you are going mad." If you do take SSRIs it can take two months to feel different and you will need to continue to see your GP while you are taking them. "I say from the start that they will need to take them for 12 months," Lester says.

If you want face-to-face CBT you may have to wait a couple of months (depending on your area). But there are things you can do to improve your mood in the meantime. Sheila Gill, a psychological therapist in north London and co-author of Therapy for Beginners says it can feel as if tackling depression is an impossible task. "When people are depressed they can debride their lives of activities, which makes them feel even worse. We try to get them up, moving about and doing things. Normally when you're tired you need to rest more but the fatigue of depression is such that going to the gym or other physical activity, even walking to the corner shop, may help you to feel better."

Doing something new, no matter how small, may help to lift your mood. "If you walk home along a different street, choose a different kind of sandwich or sit in a different chair, you may begin to feel that you can begin to change bigger things."

"I also tell people to begin to adopt an attitude of kindness towards themselves," says Gill. "In depression, the single most important thing to challenge is negative thought processes."

When you have CBT you may be surprised how much you're involved in it – there's no sitting back and being "therapised". "We give people homework such as keeping weekly thought journals. When your mood is low, your thoughts don't reflect reality. We help people to challenge their thoughts as in a court case. If you say you're a failure we will ask if there is any evidence in your life that contradicts that view. I encourage people to put a distance between themselves and their thoughts."

Part of the role of a therapist, says Gill, is to bring out your patient's inner resilience. Campbell has certainly kept his, writing extensively on depression despite having recurrent bouts of the condition. "I am actually very proud of my breakdown," he says. "Or the recovery at least. And of the fact I have sort of learned to live with depression."